1. Field of the Invention:
The present invention relates to a method for operating a scanning microscope to scan the surface of a specimen, and to a scanning microscope arrangement to implement such method.
2. Description of the Prior Art:
VLSI (very large scale integrated) circuits are usually checked by computer controlled test installations. Such tests are carried out over relatively long periods of time by the test installations. However, even such long tests are incomplete since errors can only be localized with great difficulty, even though the errors are recognized by analyzing the voltage levels at the outputs of the circuit which are dependent on bit patterns fed into the circuit during the test. Additional measurements, therefore, must be carried out for the inside of the VLSI circuits, particularly during the development phase.
Electron beam measuring methods which are utilized in all areas of development and manufacture of micro-electronic components have proven particularly suitable for the internal measurements of VLSI circuits. A modified scanning electron microscope is usually used. However, a large apparatus expenditure is required and, for components having passivation layers, a charging of the specimen surface occurs which deteriorates the precision of the measurements.
In addition to electron beam measuring installations, optical scanning microscopes, and in particular scanning laser microscopes, provide load-free operation and are therefore used for checking VLSI circuits. See, for instance, R. Mueller "Scanning Laser Microscope for Inspection of Microelectronic Devices", volume 13, No. 1, Siemens Forschungs - und Entwicklungsberichte, pages 9-14 (1984).
The structure and functions of a scanning laser microscope are also described in V. Wilke "Laser Scanning in Microscopy", proceedings of the International Society for Optical Engineering: Advances in Laser Scanning and Recording, April 19-20, 1983, Geneva, Switzerland.
To test a specimen, such as an integrated circuit, a finely focused laser beam of a scanning laser microscope is scanned point-by-point over the specimen so that the interaction of the laser beam with the specimen causes the generation of electron-hole pairs. The photocurrent, which results from the separation of the electron-hole pairs in the region of a pn-junction and which is measurable in an external circuit, is used to modulate the intensity of a write beam of a cathode ray tube which is swept across the picture screen in synchronization with the laser beam. This creates a two-dimensional illustration of the pn-junctions within the circuit.
The possible interactions of the primary beam with the specimen substance can be influenced by the scan rate. To guarantee generation of the desired physical effect by the primary beam at every location of the specimen surface, and to document the physical effect through a photocurrent, it has hitherto been necessary to scan the specimen at a very low scan rate. Particularly since micro-electronic components must be operated at low drive frequencies during testing, extremely long image recording times result.